In a radio scheme in the uplink of next generation mobile communication packet access, it is conventionally necessary to simultaneously realize a high transmission power efficiency of a terminal and high-speed data transmission in a multipath environment to expand an available communication area. Recently, a single-carrier (SC) system and a multicarrier (MC) system have been examined as a radio scheme to be determined with the above-mentioned request conditions taken into account. When the same transmission rate is realized, the single-carrier scheme performs a high-speed data transmission using one carrier at a predetermined frequency band. The multicarrier scheme divides a predetermined frequency band into a plurality of frequency bands, and performs a low-speed data transmission using a specific carrier at each frequency band.
In the packet access, an adaptive modulation scheme is used to maximize system throughput. The adaptive modulation scheme transmits data by the best effort of selecting the optimum modulation scheme depending on the propagation environment of a terminal. That is, when the propagation environment is hostile, a modulation scheme of a low modulation level is used, for example, to modulate data with errors suppressed by using phase shift keying (PSK). When the propagation environment is good, a modulation scheme of a high modulation level is used, for example, to modulate a large volume of data using a quadrature amplitude modulation (QAM). Selected, for example, is the optimum adaptive modulation scheme that measures the reception quality in an uplink from a terminal in a predetermined period by a base station, and transmits data of the terminal based on the measurement result. Then, the information about the modulation scheme (modulation mode) is transmitted to the terminal through a control channel in the downlink.
FIG. 1 shows an example of a configuration of the conventional adaptive modulation apparatus using the single-carrier scheme.
As shown in FIG. 1, the adaptive modulation apparatus using the single-carrier scheme according to the related art is configured by adaptive modulation part 101, single-carrier generation part 102, quadrature modulation part 103, and transmission amplifier 104. Adaptive modulation part 101 selects a modulation scheme in a predetermined period based on a modulation mode determined according to the reception information obtained when a signal transmitted in a wireless channel is received, and modulates a signal in the selected modulation scheme, Single-carrier generation part 102 allows the signal modulated by adaptive modulation part 101 to pass through a transmission filter, makes a symbol-waveform adjustment on the signal, and generates a single-carrier signal. Quadrature modulation part 103 performs a quadrature frequency conversion on the single-carrier signal in a baseband, and converts the signal into a carrier band signal. Transmission amplifier 104 amplifies the carrier band signal and transmits the signal to a transmission antenna.
FIG. 2 shows an example of a configuration of the conventional adaptive modulation apparatus by the multicarrier scheme.
The adaptive modulation apparatus using the multicarrier scheme according to the related art is configured by adaptive modulation part 101, multicarrier generation part 105, quadrature modulation part 103, and transmission amplifier 104. Adaptive modulation part 101 selects a modulation scheme in a predetermined period based on the modulation mode, and modulates a signal in the selected modulation scheme. Multicarrier generation part 105 divides the signal modulated by adaptive modulation part 101 into a plurality of parts, allows each of the signal parts to pass through a narrow band transmission filter, and generates a frequency divided multicarrier signal. Quadrature modulation part 103 performs a quadrature frequency conversion on the baseband multicarrier signal, and converts it into a carrier band signal. Transmission amplifier 104 amplifies the carrier band signal and outputs it to the transmission antenna. In the wireless communication system, multicarrier generation part 105 is widely used with the orthogonal frequency division multiplexing (OFDM) system capable of efficiently arranging a multicarrier signal at the shortest subcarrier frequency intervals.
In the configurations shown in FIGS. 1 and 2, only the minimal components are described, and the frequency conversion over plural stages, an amplifier, and a filter at each part are omitted.
FIG. 3 shows an example of a configuration of an OFDM transmission apparatus used as multicarrier generation part 105 shown in FIG. 2.
As shown in FIG. 3, the OFDM transmission apparatus according to the example of the configuration is configured by S/P converter 11, IDFT part 12, P/S converter 13, and GI addition part 14. S/P converter 11 performs an S/P conversion on a transmitted signal from a serial signal to a parallel signal, and divides each subcarrier into plural transmission sequences. There is a method of spreading or scrambling each subcarrier signal, but the description of the method is omitted here. IDFT part 12 performs an inverse discrete Fourier transform (IDFT) to convert all subcarrier signals into signals in a time domain, and outputs resultant signals. The signal in a time domain output from IDFT part 12 needs oversampling to remove a harmonic after an analog conversion. For example, as shown in FIG. 3, the size of IDFT part 12 is set larger than the number of subcarriers of the signal bands of the OFDM, and “0” is inserted into the harmonic part, thereby generating a signal of an oversampled time domain. As another method, the size of IDFT part 12 can be set to the number of subcarriers of the OFDM signal band to perform oversampling by the filtering process on the time domain. P/S converter 13 performs a P/S conversion on the signal converted into the time domain from a parallel signal to a serial signal, and outputs an OFDM signal rearranged in a time series. GI addition part 14 adds a guard interval (GI) to an OFDM signal rearranged in a time series to avoid the multipath interference with the previous block when the discrete Fourier transform (DFT) is performed at the reception time. Generally, a cyclic prefixing process is performed by adding the trailing data of the DFT block to the header.
In addition, there is a method of selecting one radio scheme from the above-mentioned two radio scheme s by providing a switch for switching between the single-carrier scheme and the multicarrier scheme (for example, refer to the Japanese Patent Laid-Open No. 2004-080333).
Since the adaptive modulation apparatus using the single-carrier scheme shown in FIG. 1 can set a low back-off (difference between the maximum output level at which no signal distortion occurs and the output saturation level) of transmission amplifier 104 by the low peak to average power ratio (PAPR) of a single-carrier signal, the apparatus excels in transmission power efficiency. The single-carrier scheme is congenial to a low PAPR modulation scheme, that is, a low order modulation scheme (for example, PSK), thereby making the most of the characteristic of the low PAPR of a single-carrier signal. However, the single-carrier scheme is badly degraded in reception characteristic when the QAM modulation that is poor in multipath resistance is used, thereby degrading the peak transmission rate. That is, the single-carrier scheme is not congenial to a high PAPR, that is, a high order modulation scheme (for example, QAM).
On the other hand, in the adaptive modulation apparatus using the multicarrier scheme shown in FIG. 2, since the multicarrier scheme has no effect of the multipath interference in delay of the GI length or less, a high-speed data transmission can be realized using the QAM modulation, and a higher-speed rate can be easily realized by applying MIMO (multiple input multiple output). That is, the multicarrier scheme is congenial to a high order modulation scheme. However, by a high PAPR of a multicarrier signal, the back-off of transmission amplifier 104 is to be set large regardless of the modulation scheme.
In the method described in the above-mentioned patent documents, the modulation scheme is not combined with the radio scheme.